The present invention relates to gas generating compositions of which combustion gases act as the operation gas for inflating air bags to be mounted in automobiles, aircraft or the like for the protection of the human body.
Recently, development has been carried out of non-azide gas generating compositions in place of sodium azide based compositions which have toxic problems, as gas generating compositions for air bag systems mounted in transportation means (vehicles) such as automobiles and the like. As the non-azide gas generating compositions, U.S. Pat. No. 4,909,549 discloses a composition comprising a tetrazole or triazole compound containing hydrogen, and an oxygen-containing oxidant, U.S. Pat. No. 4,369,079 discloses a composition comprising a metal salt of bitetrazole containing no hydrogen, and an oxygen-containing oxidant, and Japanese Patent Application Laid-Open (JP-A) No. 6-239683 discloses a composition comprising carbohydrazide, and an oxygen-containing oxidant.
However, when these non-azide fuels are used, essential are a large amount of metal compounds such as metal salts, metal oxides and the like as oxidants or catalysts. Even though such compositions are improved as compared with azide compositions in a view of toxic properties, they have still problems that combustion products contain mist formed of solid and liquid of metal compounds, and gas generating efficiency decreases due to formation of residues in an inflator, and therefore a large amount of gas generating compositions must be used. Further, when solid mist and liquid mist having high temperature immediately after combustion are allowed to contact directly with a bag, the bag tends to be damaged. To cut off such mist, additional parts such as a metal net-like filter and the like are consequently required. Accordingly, weight reduction and downsizing of a gas generator itself is difficult to be achieved with a gas generating composition which forms a large amount of mist and consequently exhibits low gas generating efficiency.
Use of a non-metal compound as an oxidant such as ammonium perchlorate or ammonium nitrate is advantageous for reduction of mist and improvement of gas generating efficiency because these compounds turn into gases on combustion. However, when a composition containing a large amount of ammonium perchlorate burns, a hydrochloric acid gas is generated in amount in significantly excess of allowable value for human bodies and environments. Ammonium nitrate, whose one of phase transition temperatures is in normal temperature range (about 32xc2x0 C.), changes largely in volume when passing the transition temperature. Large change in volume of a molded article leads to unstability in abilities of the composition, therefore, a composition containing ammonium nitrate showing large change in volume is not suitable to be used in environment wherein an automobile air bag is exposed to various temperature changes.
For solving such problems when ammonium nitrate is used, there is a method such that a phase stabilized agent which can suppress shift of phase transition temperature and change in volume is added to ammonium nitrate. For example, WO095/04710 discloses a gas generating composition comprising a phase stabilized ammonium nitrate, a nitrogen-containing compound such as triaminoguanidine nitrate to be used as a fuel, and an organic binder. Moreover, U.S. Pat. No. 5,545,272 and WO096/27574 disclose a gas generating composition which obtains a melting point of 100xc2x0 C. or more by using, as essential components, 35 to 55% by weight of nitroguanidine and 45 to 65% by weight of a phase stabilized ammonium nitrate.
However, such a composition has high initiation sensitivity and causes a problem that there exists constantly crisis accompanying production, transportation and other handling in large amount. Further, there are other problems of such a composition that the composition is burnt successfully in relatively high pressure range, while at lower pressure range, a pressure exponent which indicates the sensitivity of burning rate against combustion pressure is high, and in some cases, combustion is interrupted or ignition is impossible.
It is desirable for a gas generating composition for an air bag that it is safe against human bodies and environments, the gas output is high, the amount produced of solid and liquid particles (residues), namely the amount produced of metal compounds is small, and safety regarding handling such as production, transportation and the like is high, and further, it is stable against change in pressure and the like. Therefore, the known gas generating compositions as described above can not be satisfactory regarding application to air bag systems.
An object of the present invention is to provide a gas generating composition for an air bag which can enhance safety of producers and users when applied to air bag system by improving combustion behavior and handling safety, and can downsize and reduce weight of air bag system.
The present inventors have found that the above described object can be attained by the following way and completed the present invention; By combining a phase stabilized ammonium nitrate with a compound having pressure exponent adjustment effect or detonation inhibition effect as well as utilizing synergistic action with other components, the problems caused when a phase stabilized ammonium nitrate is used as an oxidant can be solved and only advantages thereof can be provided.
Namely, the present invention provide a gas generating composition for an air bag comprising (a) a guanidine derivative compound, (b) a phase stabilized ammonium nitrate and (c) a silicon compound having an activity as a pressure exponent adjuster or a detonation inhibitor.
The invention provides a gas generating apparatus which comprises the composition as defined above, an airbag system for automobiles or vehicles including the same apparatus and use of the composition as defined above in an airbag system.
In the gas generating composition for an air bag of the present invention, only advantages of the component (b), a phase stabilized ammonium nitrate can be provided by the action of the component (c), silicon compound. Consequently, a large amount of gas can be generated by combustion, and safety in handling such as production, transportation and the like can be enhanced due to low initiation sensitivity, and moreover, it can be burned successfully at lower pressure range as compared with a conventional gas generating composition containing a phase stabilized ammonium nitrate.
The gas generating composition for an air bag of the present invention can significantly downsize and reduce weight of a gas generator since generation of mist is suppressed and gas generation efficiency is enhanced on combustion by such outcome of only advantages of the component (b), a phase stabilized ammonium nitrate.
The guanidine derivative compound, i.e. the component (a) of the present invention, acts as a fuel in the composition. Such compound has high nitrogen content and low carbon content with chemically stable structure and can accomplish high speed burning in addition to reducing the amount of a poisonous gas (carbon monooxide) produced on combustion.
As the component (a), guanidine derivative compound, there are listed one or more compounds selected from the group consisting of nitroguanidine (NQ), guanidine nitrate (GN), guanidine carbonate, guanidine perchlorate, aminonitroguanidine, aminoguanidine nitrate, aminoguanidine carbonate, aminoguanidine perchlorate, diaminoguanidine nitrate, diaminoguanidine carbonate, diaminoguanidine perchlorate, triaminoguanidine nitrate and triaminoguanidine perchlorate. Among these compounds as the component (a), preferable are nitroguanidine, guanidine nitrate, aminonitroguanidine, aminoguanidine nitrate, diaminoguanidine nitrate and triaminoguanidine nitrate.
The content of the component (a) in the composition can be appropriately set depending on oxygen balance of a guanidine derivative compound, amount used of a binder, and the like, and is preferably from 5 to 60% by weight, particularly preferably from 5 to 50% by weight.
The phase stabilized ammonium nitrate which is the component (b) of the present invention is a component acting as an oxidant.
Examples of the phase stabilized agent include potassium salts such as potassium nitrate, potassium perchlorate, potassium chlorate, potassium chromate, potassium bichromate, potassium permanganate, potassium sulfate, potassium chloride, potassium fluoride and the like which are dissolved in hot water. The mixing ratio of ammonium nitrate to a phase stabilized agent can be appropriately set in the range wherein residues in burning are not practically problematical, and preferably, the amount of ammonium nitrate is from 98 to 70% by weight and the amount of a phase stabilized agent is from 2 to 30% by weight, and particularly preferably, the amount of ammonium nitrate is from 97 to 80% by weight and the amount of a phase stabilized agent is from 3 to 20% by weight.
Further, a solidification preventing agent can be compounded into a phase stabilized ammonium nitrate. As the solidification preventing agent, magnesium oxide, powder silica and the like can be listed. The compounding of solidification preventing agent is preferably from 0.05 to 2.0% by weight, particularly preferably from 0.1 to 1.0% by weight based on the phase stabilized ammonium nitrate.
The component (b), a phase stabilized ammonium nitrate can be obtained by a suitable physical treatment of a mixture of ammonium nitrate and a certain amount of agent and the like, for example, by evaporating and drying an aqueous solution of ammonium nitrate, phase stabilized agent or the like under heating, as well as other treatments.
The content of the component (b) in the composition is preferably from 40 to 90% by weight, particularly preferably from 50 to 85% by weight.
The component (c) used in the present invention is a silicon compound which has an activity as a pressure exponent adjuster or a detonation inhibitor.
As the component (c), silicon compound, there are listed one or more compounds selected from the group consisting of silicon nitride, silicone, silicon carbide, silicon dioxide, silicates and, clay minerals of silicates (kaoline, acid clay, bentonite and the like) and the like.
The content of the component (c) in the composition is preferably from 0.3 to 10% by weight, particularly preferably from 0.5 to 7% by weight. When the content of the component (c) is 0.3% by weight or more, initiation sensitivity can be reduced to enhance safety in handling, and further, burning can be conducted stably even under lower pressure. When the content of the component (c) is 10% by weight or less, production cost can be reduced while maintaining the above-described properties.
Into the gas generating composition for an air bag of the present invention, a combustion promoter can be further compounded in the range wherein thermal stability and mechanical properties of the composition are practically permissible. As the combustion promoter, one or more compounds are listed selected from the group consisting of metal oxides, ferrocenes, carbon black, sodium barbiturate, ammonium bichromate, potassium bichromate and the like. As the metal oxide, there are listed copper oxide, cobalt oxide, iron oxide, manganese oxide, nickel oxide, chromium oxide, vanadium oxide, molybdenum oxide or complex metal oxides thereof.
The amount compounded of the combustion promoter based on the composition is preferably from 0.05 to 5% by weight, particularly preferably 0.1 to 4% by weight.
Into the gas generating composition for an air bag of the present invention, one or more compounds selected from energetic binders and non-energetic binders can be compounded depending on increase in strength or molding ability of a gas generating agent.
Examples of the non-energetic binder include sodium carboxymethylcellulose (CMC), cellulose acetate (CA), cellulose acetate butyrate (CAB), methylcellulose (MC), hydroxyethylcellulose (HEC), polyvinylpyrrolidone (PVP) polyvinyl alcohol (PVA) or modified products thereof, polyacrylamide (PAA), polyacrylhydrazide (APAH), hydroxy terminated polybutadiene (HTPB), carboxy terminated polybutadiene (CTPB), polycarbonate, polyester, polyether, polysuccinate, polyurethane, thermoplastic rubbers, silicones and the like.
Examples of the energetic binder include azidemethylmethyloxetane, glycidyl azide polymer (GAP), polymer of 3,3-bis(azidemethyl)oxymethane, polymer of 3-nitratemethyl-3-methyloxymethane, nitrocellulose and the like.
The amount compounded of the binder component based on the composition can be appropriately set depending on molding property required for the composition, and the like, and is preferably from 2 to 25% by weight, particularly preferably from 5 to 20% by weight.
For producing the gas generating composition for an air bag of the present invention, a wet method in which mixing is conducted in the presence of water, organic solvent and the like can be used in addition to a dry method in which the components (a), (b) and (c) and the like are mixed in powder condition. Further, the composition can be compression-molded into a pellet using a tablet machine or the composition can be compression-molded into a disk using a disk molding machine. Furthermore, a pellet and disk can be ground or made into a granule using a granulator, or the composition can be extrusion-molded into an extruded agent (non-porous, single-holed, porous) using an extruder (extrusion molding machine).
In relation to the gas generating composition for an air bag of the present invention, the following formula (1) showing sensitivity of burning rate against burning pressure:
r=aPnxe2x80x83xe2x80x83(1)
[wherein, xe2x80x9crxe2x80x9d represents a burning rate, xe2x80x9cPxe2x80x9d represents a burning pressure, xe2x80x9caxe2x80x9d represents a constant varying depending on the kind of the gas generating composition and the initial temperature, and xe2x80x9cnxe2x80x9drepresents a pressure exponent.]
This formula determines that xe2x80x9cnxe2x80x9d representing a pressure exponent at a burning pressure (P) of 50 to 70 kg/cm2 is preferably of 0.95 or less, particularly preferably of 0.9 or less.
It is preferable that the gas generating composition for an air bag of the present invention is not judged as detonation by a detonator test (plastic rainpipe test) according to Japan Explosive Society standard ES-32. The detonator initiation test represents the sensitivity of explosives or explosive substances to the detonation shock caused by a detonator, therefore, due to reduction of initiation sensitivity, namely, judging of no-detonation in the above-described test, not only handling safety in production and use but also safety in all handling such as storage, transportation and the like can be improved.